ABSTRACT The hallmark of systemic lupus erythematosus (SLE) is the production of antibodies to nuclear antigens such as ribonucleoproteins and DNA. Antibodies to double-stranded DNA (dsDNA) and/or chromatin represent an important transition from benign to overt clinical SLE and may predict flares and the severity of tissue damage. The mechanisms of tolerance to chromatin/DNA and of its breakdown in SLE are poorly understood. We explored these mechanisms by focusing on DNASE1L3, a unique secreted DNase whose null mutations are associated with an early-onset familial SLE. Our studies showed that chromatin in microparticles derived from apoptotic cells represented a specific substrate of DNASE1L3 in vitro and in vivo. They also demonstrated that the chromatin and/or other antigens were exposed on the surface of microparticles and recognized by autoantibodies in a DNASE1L3-sensitive manner. We hypothesize that DNASE1L3-sensitive DNA/protein complexes on microparticles are important self-antigens in human SLE, and that that DNASE1L3 can be used to target them for therapeutic purposes. The collaborative nature of the project and unique patient samples and technologies available from our co-investigators will be leveraged to develop and test this hypothesis. In Aim 1, the incidence and clinical relevance of antibodies to DNASE1L3-sensitive antigens in SLE patients will be explored. In Aim 2, DNASE1L3-sensitive antigens on microparticles and the antibodies targeting them will be characterized at the molecular level. In Aim 3, the utility of DNASE1L3 as a therapeutic agent will be tested in animal models of SLE. Collectively, these studies would provide insights into the origin and mechanisms of the pathogenic responses to DNA and associated antigens in human SLE, and facilitate novel approaches towards their therapeutic blockade.